I want to change only the sound of an instrument, not the notation. How do I do that?

I am arranging choir pieces but I am not happy with the standard "ahh" sounds for voices, so I want to change the instrument to sound like for example an electrical piano. However when I change instrument, I also change the notation and clef, and it makes a piano clef. How do I change just the sound and not the instrument so it keeps notated as SATB but with sounds of another instrument?